1. Field of the Invention
The present invention relates to a thin film transistor, and more specifically, to a thin film transistor for increasing the conductivity of a channel region and suppressing the leakage current of a back channel region.
2. Discussion of the Related Art
In accordance with the increased demand for various forms of display devices for information-dependent users, a great deal of research is being actively conducted on flat display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electro luminescent displays (ELDs), field emission displays (FEDs), and vacuum fluorescent displays (VFDs).
A thin film transistor (TFT) having a channel region provided on a semiconductor layer arranged on a substrate having an insulating surface is widely utilized as a switching device of each pixel constituting a display device such as an LCD.
Current-voltage properties of the thin film transistor generally used for display devices is affected by factors such as the characteristics of the semiconductor layer and the gate insulating film in the thin film transistor, the interface properties between the gate insulating film and the semiconductor layer, ohmic contact properties between the semiconductor layer and the source-drain electrode in the thin film transistor, and field effect electron mobility.
When a voltage not lower than a threshold voltage is applied to the gate electrode of the thin film transistor, mobile carriers (free electrons) are induced to the semiconductor layer by field effect, and are moved due to a bias voltage formed between the source and drain electrodes, whereby the electric current flows through the thin film transistor which then enters an on-state.
The semiconductor layer of the thin film transistor is generally made of a semiconductor material such as amorphous silicon or polysilicon. As the semiconductor layer is made of amorphous silicon, realizing circuits operating at a high speed is disadvantageously difficult due to low mobility. The semiconductor layer made of polysilicon has high mobility, but disadvantageously requires additional compensation circuits due to a non-uniform threshold voltage.
Conventional methods for fabricating a thin film transistor using low temperature poly-silicon (LTPS) are inapplicable to large substrates due to an expensive process such as laser thermal treatment, and the difficulty associated with controlling the characteristics of the thin film transistor. In order to address these problems, research into the use of oxide for a semiconductor layer is currently underway.
An oxide semiconductor layer generally exhibits superior mobility and high Ion/Ioff ratio, as compared to the silicon semiconductor layer. However, when the semiconductor layer is made entirely of oxide, the conductivity of a channel region thereof increases, and at the same time, the conductivity of a back channel thereof also increases. That is, when the oxide is used to form the entire semiconductor layer, the channel region and the back channel region of the semiconductor layer are made of the same composition. For this reason, as the conductivity in the channel region increases, the leakage current through the leakage channel generated in the back channel region in the off-state increases. On the other hand, if the conductivity were to be decreased in order to suppress the leakage current in the back channel region, the conductivity in the channel region also decreases, thus causing deterioration in the properties of the thin film transistor.
As such, when the semiconductor layer is made entirely of the same material such as oxide, a trade off is thus made between the channel region and the back channel region, which limits the improvement operation properties of the thin film transistor having such semiconductor layer.